Heavy oil, extra-heavy oil and bitumen (herein collectively “heavy oil”) cannot be transported by pipeline in a raw state due to a very high viscosity and density. Currently there are two options to make a heavy oil feedstock transportable, for instance by pipeline to refineries. In one option, a diluent is added to heavy oil to reduce the viscosity and the density of the blend to a value meeting the requirements for pipeline transport. Typically about one volume of diluent is required for between two and three volumes of heavy oil, so significant pipeline capacity is taken up by the diluent. The diluent must then be separated at the receiving refinery. In a second option, the heavy oil feedstock is upgraded to synthetic crude oil (SCO), which can then be processed directly in refineries. Upgrading occurs when the carbon number of the heavy oil is shifted from an average of 25 to 30 for each molecule to about 7 to 15 in the upgraded product. At the same time, the hydrogen-to-carbon ratio is increased from between about 1.3 and 1.5 in the heavy oil to between about 1.6 and 2.2 in the upgraded product.
In practice, heavy oil can be upgraded to improve the hydrogen-to-carbon ratio according to two routes. The first involves the rejection of carbon and the second involves the addition of hydrogen. FIG. 1 shows exemplary schemes associated with these prior art upgrading efforts, which are briefly described below.
Processes which are based on coking and de-asphalting of heavy oil (i.e., carbon rejection) suffer from product loss and low yield. In coking processes, carbon losses to coke and asphaltenes may account for over 20% (m/m) of the feed which amounts to a considerable loss of product, considering that the product still requires further refining. Solvent requirements in de-asphalting processes and the high amount of energy required to separate the solvent from de-asphalted oil also add considerable costs. Examples of carbon rejection processes include the CCU Process by UOP, the JetShear process by Fractal Systems Inc., and the WRITE process by Western Research Institute. Some carbon rejection processes overcome the poor conversion efficiencies by gasifying the coke co-product to produce a synthesis gas that can be used for process heat or can be converted into liquid hydrocarbons by, for example, Fischer Tropsch synthesis. The FT-Crude process is an example of this process. This approach results in a complex process flowsheet and high capital costs.
Hydrogen addition processes are based on hydrocracking in the presence of a suitable catalyst. The purpose of the catalyst is to activate the addition of hydrogen and kinetically suppress the formation of gases and coke. The majority of hydrogen addition processes utilize catalysts formulated from metals in the columns 6, 8, 9 and 10 of the Periodic Table. These catalysts are tailored for selective conversion and high activity in order to maximize process throughput and product quality. Challenges associated with selective and high activity catalysts are rapid deactivation, high costs, and complex catalyst preparation, handling and recovery procedures. The reactors used are designed primarily to manage the handling of the catalysts in an effective way. In so doing, the reactors suffer from excessive capital costs, a narrow range of operating conditions and high maintenance. As a consequence of the need for effective catalyst management, hydrocracking processes are defined by the type of reactor used. There are two main reactor types used for hydrocracking, namely fixed bed reactors and fluidized bed reactors.
Fixed-bed reactors have been used to hydrocrack residues containing low concentrations of metals. In many cases, the operation of fixed bed reactors is severely inhibited by the rapid deactivation of the catalyst which results in high operating pressure, low conversion, uneven temperature distribution, and poor quality products. The low catalyst cycle time makes fixed bed processes capital intensive with limited overall benefits.
There are several types of fluidized bed reactors that can be used. Examples are ebullated bed reactors and bubble column reactors. Ebullated bed reactors are suited to the three-phase mixing of gases, liquids and solids, where mixing results from the upward flow of gas and liquid, that also results in the formation of an expanded catalyst bed. The catalysts are generally particles with sizes that fall into the millimeter domain. Ebullated bed reactors allow the handling of higher amounts of metals and fine solids in the feed as the catalyst is easily replaced. However when using supported metal catalysts, these reactors suffer from poor conversion of asphaltenes and the formation of sediments or sludge. This is due primarily to limited mass transfer in the catalyst pores. Other disadvantages associated with these reactors include firstly, the narrow range of gas flow rates required to maintain the catalyst particles in a fluidized condition; and secondly, a limited liquid residence time due to the high gas holdup required for fluidization.
An improvement on supported metal catalyst in fluidized bed reactors is the use of dispersed catalysts, which are colloidal suspensions of nano-sized catalytic particles. This improvement typically takes the form of a slurry comprised of oil and finely dispersed catalyst (typically a transition metal sulphide such as Mo or W) which is fed into a hydrocracking reactor. The high density of available reaction sites avoids the plugging of pores that causes de-activation of supported metal catalysts. However, maintaining uniform dispersion of the catalyst particles remains a challenge, and has typically been limited to hydrogen induced mixing in bubble column reactors.
Slurry hydroconversion processes using bubble column and ebullated bed reactors have been applied to the upgrading of heavy oil and bitumen with the objective of producing a bottomless SCO that is characterized by an API gravity of at least 30°, removal of sulphur and heteroatoms, and a reduction in viscosity. Examples of upgrading processes that utilize packed bed, ebullated bed or bubble column reactors are the Eni Slurry Technology (EST) by Eni S.p.A., the HCAT Process by Headwaters Technology Innovation, the Uniflex Process by UOP, Veba Combi-Cracking (VCC) by BP and KBR and the HDH Process by PDVSA.
In contrast to producing a SCO by upgrading, partial upgrading of heavy oil and bitumen seeks to produce an oil product with an API gravity above about 19° (for example, between 20° and 30°), a viscosity less than about 350 cSt (at 7.5° C.), and a partial reduction in the concentration of sulphur and other heteroatoms. This partially upgraded crude product may then be transported, for example by pipeline, to a refinery for further processing.
The use of bubble column or ebullated bed reactors in a partial upgrading process presents a challenge due to the low margins associated with the partially upgraded products, the high capital intensity and high operating costs. Examples of recent patents that teach a method of partial upgrading of heavy oil and bitumen through slurry hydroconversion are shown below.
In U.S. Pat. Nos. 6,096,192 and 6,355,159, a two-step method is used to produce a pipeline-ready oil. The heavy hydrocarbon is treated by a slurry hydroconversion process in the presence of phosphomolybdic acid at a concentration of between 150 and 500 ppm or coke-derived fly ash catalyst (between 0.5 and 5% (m/m)), under a pressure and temperature in the range of 48 to 103 bar and 400 to 450° C. The oil produced in this manner still does not meet pipeline specifications and therefore requires further mixing with sufficient diluent to meet the pipeline specifications.
In U.S. Pat. No. 4,485,004, a process for upgrading heavy oil and bitumen is taught in which a slurry of the heavy hydrocarbon, a hydrogen donor solvent (such as tetralin), and a particulate hydroconversion catalyst (such as Co, Mo, Ni, W or spent hydrodesulphurization catalyst) is treated under hydrogen. Typical operating conditions include a pressure and temperature in the range of 110 to 170 bar and 400 to 450° C., a catalyst concentration in the range of 3 and 5% (m/m) and a residence time between 2 and 3.5 h.